The present study has some limitations. First, patients in the current sample were treated as part of a randomized control trial, which maximizes internal validity at the expense of external va- lidity or generalizability. Random assignment and rigid exclusion
Criteria may introduce differences in symptom severity, motivation, and baseline personality characteristics; research comparing depressed patients in a randomized control trial to those in a clinic-based study, however, observed few such differences (Kushneretal.,2009). Nevertheless, results from the current study suggest that patients who prematurely terminated treatment had higher depression severity and neuroticism scores, and lower agreeableness. Second, the lack of conditional indirect effects from posthoc multigroup mediation models may be due to insufficient statistical power to detect significant mediation effects within each treatment condition(FritzandMacKinnon,2007). Future studies that are sufficiently powered might further examine intervention-specific mediation results. Nevertheless, therapeutic alliance positively predicts outcome regardless of treatment
modality (Krupnick etal.,1996). A final limitation is that we did not assess the individual characteristics of clinicians in this study, and thereby did not examine the impact of clinician characteristics and patient-clinician interactions. These represent important variables for future research, as both may account for variance in the development of therapeutic alliance and its impact on treat- ment outcome (DeRubeis et al., 2005).